This invention relates to cardiac stimulating devices and particularly to implantable cardiac stimulating devices, including implantable cardiac pacemakers and implantable cardiac defibrillators, as well as implantable cardioverters and cardioverter/defibrillators. More particularly, this invention relates to a method and apparatus for compensating for the tolerance in a capacitor in such a cardiac stimulating device.
An implantable cardiac stimulating device is designed to deliver electrical shocks, or "pulses," of varying energy content to the heart of a patient in whom it is implanted. The magnitude of the electrical pulse is relatively small in a pacemaker, relatively large in a defibrillator, and somewhere in between in a cardioverter. The magnitude of the electrical pulse to be delivered is calibrated in terms of energy, usually in joules. For example, a pacemaker might deliver a pulse on the order of about 25 .mu.joules to about 30 .mu.joules, while a defibrillator might deliver a pulse on the order of about 30 joules to about 40 joules.
The cardiac stimulating pulse is usually produced by discharging a capacitor in which the desired amount of energy has been stored. The amount energy E stored in a capacitor having a capacitance C is determined by the well known expression: EQU E=0.5CV.sup.2, (1)
where V is the potential to which the capacitor is charged.
However, capacitors used in implantable cardiac stimulating devices may vary from their nominal capacitance. Indeed, the most commonly used capacitors in such devices are electrolytic capacitors, because of their higher capacitances. Such capacitors need periodic (e.g., every 3-4 months) "re-forming"--a full charge and discharge cycle--to maintain their capacitance. Electrolytic capacitors can initially vary from their nominal capacitances by as much as +10%/-20%, and their capacitance further changes with time. As a result, the stored energy can vary by as much as .+-.20% from the calculated value, so that the pulse delivered to the patient may be either too small to have its desired clinical effect, or too large, wasting power.
In an implantable cardiac stimulating device, which ideally should not have to be replaced, it is desirable that appropriately sized pulses be delivered, both to achieve the desired clinical result in the patient and to avoid premature battery depletion which would necessitate early replacement. Therefore, accurate calibration of pulse energy is desirable.
It would be desirable to be able to determine the true capacitance of a capacitor in an implantable cardiac stimulating device, or to otherwise compensate for variations in capacitance.
It would also be desirable to be able to determine the capacitance, or to compensate for variations in capacitance, without having to provide additional components or using additional battery power.